Gas sensor and method for controlling the same

ABSTRACT

Disclosed is a gas sensor for controlling a concentration of oxygen and/or measuring NOx by allowing a current to flow through an ion-conductive member for conducting oxygen ion, by the aid of a current supply circuit, wherein the current, which is outputted from the current supply circuit, has a pulse waveform (current signal) having a constant crest value, and the current supply circuit comprises a rectangular wave-generating circuit for controlling a frequency of the current signal on the basis of an electromotive force generated in the ion-conductive member to which the current signal is supplied. Accordingly, it is possible to highly accurately measure the predetermined gas component while scarcely being affected by the electric noise or the like.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 10/608,084,filed Jun. 27, 2003, now U.S. Pat. No. 7,160,435, which is a division ofU.S. application Ser. No. 09/598,811, filed Jun. 21, 2000, now U.S. Pat.No. 6,623,618, which is a continuation-in-part of U.S. application Ser.No. 09/213,981, filed Dec. 17, 1998, now abandoned, the entireties ofwhich are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a gas sensor and a method forcontrolling the same for measuring oxides such as NO, NO₂, SO₂, CO₂, andH₂O contained in, for example, atmospheric air and exhaust gasdischarged from vehicles or automobiles, and inflammable gases such asCO and CnHm.

2. Description of the Related Art

Recently, an oxygen sensor is widely known, for measuring a specifiedgas component, for example, oxygen, in which the voltage or the currentis controlled to apply it to an oxygen pump based on the use of anoxygen ion-conductive member composed of a solid electrolyte of ZrO₂ sothat oxygen is pumped in or pumped out under a predetermined diffusionresistance to measure a limiting current obtained during this process(see, for example, Japanese Laid-Open Patent Publication No. 8-271476).

Another sensor is also known, in which a proton pump is constructed byusing an oxygen-proton ion-conductive member so that the limitingcurrent is measured on the basis of the same principle as that used inthe oxygen sensor to measure H₂ and H₂O.

A NOx sensor 200 as shown in FIG. 15 is also known, which is used tomeasure, for example, NOx as a specified gas component.

The NOx sensor 200 is operated as follows. That is, a measurement gas isintroduced into a first hollow space 204 via a first diffusionrate-determining section 202. A first oxygen-pumping means 212, which isconstructed by an inner pumping electrode 206, an oxygen ion-conductivemember 210, and an outer pumping electrode 208, is used to pump out orpump in oxygen contained in the measurement gas in such a degree thatthe measurement gas is not decomposed. Subsequently, the measurement gasis introduced into a second hollow space 216 via a second diffusionrate-determining section 214. A second oxygen-pumping means 226, whichis constructed by a measurement gas-decomposing electrode 218 arrangedin the second hollow space 216, an oxygen ion-conductive member 220, anda reference electrode 224 arranged in a reference air section 222, isused to pump out oxygen which is produced by decomposition effected bythe catalytic action of the measurement gas-decomposing electrode 218.The sensor measures the value of current which is required to pump outthe oxygen.

In other words, the foregoing gas sensors are operated such that thespecified gas component is detected by using the ionic current, and theconcentration of the predetermined gas is ensured in the internal spaceof the sensor by controlling the ionic current value.

However, the gas sensors as described above are disadvantageous asfollows. That is, when the concentration of the measurement gas is low,the pumping current is decreased. As a result, it is difficult toperform the detection in some cases, and the accuracy is greatlydeteriorated by the external electric noise in other cases.

For example, in the case of the NOx sensor 200 shown in FIG. 15, whenthe NOx concentration in the measurement gas is 10 ppm, the signal levelis in a degree of about 0.05 μA. As a result, it is difficult to performthe detection. Further, it is feared that the measurement accuracy isgreatly deteriorated due to the external electric noise.

In order to accurately control the oxygen concentration in the secondhollow space 216, the present applicant has suggested a NOx sensor 250as shown in FIG. 16. The NOx sensor 250 comprises an auxiliary pump 252which is provided for the second hollow space 216. The controlled oxygenconcentration in the first hollow space 204 is corrected so that thecurrent, which flows through the auxiliary pump 252, is constant (see,for example, Japanese Laid-Open Patent Publication No. 9-113484 andEuropean Patent Publication No. 0 807 818 A2).

In the case of the NOx sensor 250, the auxiliary pumping current is notmore than several μA which is small. Therefore, it has been revealedthat the controlled oxygen concentration in the second hollow space 216cannot be corrected at the desire of a user in some cases.

On the other hand, in the case of the sensors as described above, thelimiting current is utilized to control the concentration of the gascomponent and measure the concentration thereof. Therefore, if thelimiting current value is changed, the output is changed. In thiscontext, for example, the limiting current value involves dispersionamong individual sensors. At present, in order to correct the dispersionamong individual sensors, a shunt resistor is provided, or a voltagedivider resistor is provided.

FIG. 17 shows an arrangement of such a countermeasure. When the current,which flows to an oxygen pump 260, is detected by using acurrent-detecting resistor Ra, the current supply from a variable powersource 262 to the oxygen pump 260 is shunted by the aid of an adjustingresistor Rb (shunt resistor).

For example, when the gas sensor has a large limiting current, the shuntresistor Rb is decreased so that the amount of shunt is increased. Thus,the amount of current, which is detected by the current-detectingresistor Ra, is decreased to be a predetermined value. On the contrary,when the gas sensor has a small limiting current, the amount of shunt isdecreased so that the current, which is detected by thecurrent-detecting resistor Ra, is adjusted to be the predeterminedvalue.

Another method is also available such that the voltage, which isgenerated between the both terminals of the current-detecting resistorRa, is subjected to voltage division by using a voltage divider circuitto obtain a predetermined output voltage.

However, when the foregoing methods (the shunt resistor system and thevoltage divider resistor system) are adopted, one extra lead wire isrequired, in accordance with which it is necessary to use a multipleterminal connector system for connecting the control circuit and thesensor, resulting in a problem concerning the cost.

SUMMARY OF THE INVENTION

The present invention has been made considering the problems asdescribed above, an object of which is to provide a gas sensor and amethod for controlling the gas sensor which make it possible to highlyaccurately measure a predetermined gas component while scarcely beingaffected by the electric noise or the like.

Another object of the present invention is to provide a gas sensor and amethod for controlling the gas sensor which are advantageous in view ofthe production cost and which make it possible to compensate thedispersion among individual sensors without increasing the number ofterminals, in addition to the requirement described above.

A gas sensor according to the present invention comprising:

a main pumping means for pumping-processing oxygen contained in ameasurement gas introduced from external space, comprising solidelectrolyte contacting with said external space, and an inner pumpingelectrode and an outer pumping electrode formed on inner and outersurfaces of said solid electrolyte; and

a measuring pumping means for decomposing a predetermined gas componentcontained in said measurement gas after being pumping-processed by saidmain pumping means by the aid of a catalytic action and/or electrolysis,and pumping-processing oxygen produced by said decomposition via saidouter pumping electrode of said main pumping means, wherein:

a concentration of oxygen is controlled and/or the predetermined gascomponent is measured by allowing a pulse-shaped current to flow throughsaid measuring pumping means;

the gas sensor further comprising:

a electromotive force-measuring circuit for constantly measuring theelectromotive force corresponding to a difference between an amount ofoxygen produced by said decomposition of said predetermined gascomponent and an amount of oxygen contained in a reference gas;

a frequency control means for controlling a frequency of saidpulse-shaped current corresponding to a difference between an theelectromotive force measured by said electromotive force-measuringcircuit and a comparing voltage; and

a measuring circuit for at least converting the frequency of thepulse-shaped current into a concentration of said predetermined gascomponent.

Accordingly, the concentration of oxygen is controlled and/or thepredetermined gas component is measured by supplying the current fromthe current supply means to the measuring pumping means.

Usually, the following method is adopted in relation to the measurementof the predetermined gas component. That is, a constant voltage isapplied to the measuring pumping means to measure the predetermined gascomponent by detecting the value of current flowing through themeasuring pumping means depending on the amount of oxygen during thisprocess. In such an ordinary method, the detected current value isextremely small. Therefore, a problem arises in that the measurementtends to be affected by the external electric noise.

On the contrary, according to the present invention, the current, whichis supplied to the measuring pumping means, has the pulse waveformhaving the constant crest value. Further, the frequency of the pulsewaveform is controlled. In such a procedure, the use of the pulsewaveform makes it possible to obtain the crest value which is higherthan those obtained when the current is supplied in the direct currentform. Therefore, it is possible to allow the system to be scarcelyaffected by the electric noise or the like. The use of the frequency asthe measured value makes it possible to increase the output dynamicrange (frequency region) with respect to the inputted electromotiveforce. Thus, it is also possible to improve the measurement sensitivity.

It is preferable for the gas sensor constructed as described above thata power source for the current supply means is a constant voltage powersource, and a resistor is connected in series to a current supply lineto the measuring pumping means. In this arrangement, the voltage fromthe constant voltage power source is allowed to have a pulse-shapedvoltage waveform by the aid of the current supply means. The current,which is supplied to the measuring pumping means, is a pulse-shapedcurrent which has a crest value obtained by dividing the crest value ofthe voltage by the resistance value of the series resistor. In otherwords, the crest value of the pulse-shaped current supplied to themeasuring pumping means can be adjusted by changing the resistance valueof the series resistor. In this arrangement, it is preferable that theseries resistor is selected or adjusted depending on performance of asensor element.

Accordingly, it is possible to compensate the dispersion (the dispersionconcerning the crest value and the output) among the individual sensorswithout increasing the number of terminals, which is advantageous inview of the production cost.

The gas sensor according to the present invention is preferably used fora NOx sensor for measuring NOx in a measurement gas.

In another aspect, a gas sensor according to the present inventioncomprising:

a electromotive force-measuring circuit for constantly measuring theelectromotive force corresponding to a difference between an amount ofoxygen produced by said decomposition of said predetermined gascomponent and an amount of oxygen contained in a reference gas;

a duty ratio control means for controlling a duty ratio of saidpulse-shaped current corresponding to a difference between an theelectromotive force measured by said electromotive force-measuringcircuit and a comparing voltage; and

a measuring circuit for at least converting the duty ratio of thepulse-shaped current into a concentration of said predetermined gascomponent.

According to the present invention, the current, which is supplied tothe measuring pumping means, has the pulse waveform having the constantcrest value. Further, the duty ratio of the pulse waveform iscontrolled. Also in this aspect, the use of the pulse waveform makes itpossible to obtain the crest value which is higher than those obtainedwhen the current is supplied in the direct current form. Therefore, itis possible to allow the system to be scarcely affected by the electricnoise or the like. The use of the pulse width of each waveform as themeasured value makes it possible to increase the output dynamic rangewith respect to the inputted electromotive force. Thus, it is alsopossible to improve the measurement sensitivity.

It is preferable for the gas sensor constructed as described above thata power source for the current supply means is a constant voltage powersource, and a resistor is connected in series to a current supply lineto the measuring pumping means. In this arrangement, it is preferablethat the series resistor is selected or adjusted depending onperformance of a sensor element. Accordingly, it is possible tocompensate the dispersion among the individual sensors withoutincreasing the number of terminals, which is advantageous in view of theproduction cost.

The gas sensor according to the present invention is also preferablyused for a NOx sensor for measuring NOx in a measurement gas.

In still another aspect, a gas sensor according to the present inventioncomprising:

a electromotive force-measuring circuit for constantly measuring theelectromotive force corresponding to a difference between an amount ofoxygen produced by said decomposition of said predetermined gascomponent and an amount of oxygen contained in a reference gas;

a crest value control means for controlling a crest value of saidpulse-shaped current corresponding to a difference between an theelectromotive force measured by said electromotive force-measuringcircuit and a comparing voltage; and

a measuring circuit for at least converting the crest value of thepulse-shaped current into a concentration of said predetermined gascomponent.

According to the present invention, the current, which is supplied tothe measuring pumping means, has the pulse waveform. Further, the crestvalue of the pulse waveform is controlled. Also in this aspect, it ispossible to obtain the crest value which is higher than those obtainedwhen the current is supplied in the direct current form. Therefore, itis possible to allow the system to be scarcely affected by the electricnoise or the like. As a result, it is possible to increase the outputdynamic range with respect to the inputted electromotive force. Thus, itis also possible to improve the measurement sensitivity. When the crestvalue is detected, it is also preferable that the current having thepulse waveform is converted into a voltage to perform the detection.

It is preferable for the gas sensor constructed as described above thata resistor is connected in series to a current supply line to themeasuring pumping means. In this embodiment, it is preferable that theseries resistor is selected or adjusted depending on performance of asensor element. Accordingly, it is possible to compensate the dispersionamong the individual sensors without increasing the number of terminals,which is advantageous in view of the production cost. The gas sensoraccording to the present invention is also preferably used for a NOxsensor for measuring NOx in a measurement gas.

In still another aspect, a method for controlling a gas sensor accordingto the present invention, the gas sensor comprising:

a main pumping means for pumping-processing oxygen contained in ameasurement gas introduced from external space, comprising solidelectrolyte contacting with said external space, and an inner pumpingelectrode and an outer pumping electrode formed on inner and outersurfaces of said solid electrolyte; and

a measuring pumping means for decomposing a predetermined gas componentcontained in said measurement gas after being pumping-processed by saidmain pumping means by the aid of a catalytic action and/or electrolysis,and pumping-processing oxygen produced by said decomposition via saidouter pumping electrode of said main pumping means;

wherein a concentration of oxygen is controlled and/or the predeterminedgas component is measured by allowing a pulse-shaped current to flowthrough said measuring pumping means;

wherein the method for controlling the gas sensor comprises the stepsof:

measuring constantly the electromotive force corresponding to adifference between an amount of oxygen produced by said decomposition ofsaid predetermined gas component and an amount of oxygen contained in areference gas;

controlling a frequency of said pulse-shaped current corresponding to adifference between an the electromotive force measured by saidelectromotive force-measuring circuit and a comparing voltage; and

converting at least the frequency of the pulse-shaped current into aconcentration of said predetermined gas component.

In still another aspect, the present invention lies in a method forcontrolling a gas sensor as described above, comprises the steps of:

measuring constantly the electromotive force corresponding to adifference between an amount of oxygen produced by said decomposition ofsaid predetermined gas component and an amount of oxygen contained in areference gas;

controlling a duty ratio of said pulse-shaped current corresponding to adifference between an the electromotive force measured by saidelectromotive force-measuring circuit and a comparing voltage; and

converting at least the duty ratio of the pulse-shaped current into aconcentration of said predetermined gas component.

In still another aspect, the present invention lies in a method forcontrolling a gas sensor as described above, comprises the steps of:

measuring constantly the electromotive force corresponding to adifference between an amount of oxygen produced by said decomposition ofsaid predetermined gas component and an amount of oxygen contained in areference gas;

controlling a crest value of said pulse-shaped current corresponding toa difference between an the electromotive force measured by saidelectromotive force-measuring circuit and a comparing voltage; and

converting at least the crest value of the pulse-shaped current into aconcentration of said predetermined gas component.

According to the methods for controlling the gas sensors concerning theinventions described above, it is possible to allow the system to bescarcely affected by the electric noise or the like. Thus, it ispossible to measure the predetermined gas component highly accurately.Further, it is possible to compensate the dispersion among theindividual sensors without increasing the number of terminals, which isadvantageous in view of the production cost.

The methods for controlling the gas sensors described above arepreferably applicable to a NOx sensor for measuring NOx in a measurementgas.

The above and other objects, features, and advantages of the presentinvention will become more apparent from the following description whentaken in conjunction with the accompanying drawings in which a preferredembodiment of the present invention is shown by way of illustrativeexample.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a plan view illustrating a structure of a gas sensoraccording to a first embodiment;

FIG. 2 shows a sectional view (a sectional view taken along a line II-IIshown in FIG. 1) illustrating the structure of the gas sensor accordingto the first embodiment;

FIG. 3 shows a block diagram illustrating a circuit system of the gassensor according to the first embodiment;

FIG. 4 shows a characteristic curve illustrating the change in frequencyof a pulse-shaped current signal with respect to the change in NOconcentration concerning the gas sensor according to the firstembodiment;

FIG. 5 shows waveforms illustrating a waveform of the pulse-shapedcurrent signal used for the gas sensor according to the firstembodiment, together with a direct current waveform;

FIG. 6 shows a block diagram illustrating a circuit system of a gassensor according to a modified embodiment of the first embodiment;

FIG. 7 shows a block diagram illustrating a circuit system of a gassensor according to a second embodiment;

FIG. 8 shows a characteristic curve illustrating the change in dutyratio of a pulse-shaped current signal with respect to the change in NOconcentration concerning the gas sensor according to the secondembodiment;

FIG. 9 shows waveforms illustrating a waveform of the pulse-shapedcurrent signal used for the gas sensor according to the secondembodiment, together with a direct current waveform;

FIG. 10 shows a block diagram illustrating a circuit system of a gassensor according to a modified embodiment of the second embodiment;

FIG. 11 shows a block diagram illustrating a circuit system of a gassensor according to a third embodiment;

FIG. 12 shows a characteristic curve illustrating the change in voltageof a pulse-shaped driving signal with respect to the change in NOconcentration concerning the gas sensor according to the thirdembodiment;

FIG. 13 shows waveforms illustrating a waveform of the pulse-shapeddriving signal used for the gas sensor according to the thirdembodiment, together with a direct current voltage waveform;

FIG. 14 shows a block diagram illustrating a circuit system of a gassensor according to a modified embodiment of the third embodiment;

FIG. 15 shows a sectional view illustrating a structure of anillustrative conventional gas sensor;

FIG. 16 shows a sectional view illustrating a structure of anillustrative suggested gas sensor; and

FIG. 17 illustrates an arrangement of a conventional shunt resistorsystem.

DETAILED DESCRIPTION OF THE INVENTION

Explanation will be made below with reference to FIGS. 1 to 14 forseveral illustrative embodiments in which the gas sensor and the methodfor controlling the same according to the present invention are appliedto gas sensors for measuring oxides such as NO, NO₂, SO₂, CO₂, and H₂Ocontained in, for example, atmospheric air and exhaust gas dischargedfrom vehicles or automobiles and inflammable gases such as CO and CnHm.

At first, as shown in FIGS. 1 and 2, a gas sensor 10A according to afirst embodiment comprises a main sensor device 12 which is constructedto have a lengthy plate-shaped configuration as a whole, and a currentsupply circuit 14 for supplying a pulse-shaped current signal to themain sensor device 12.

The main sensor device 12 comprises, for example, six stacked solidelectrolyte layers 16 a to 16 f composed of ceramics based on the use ofoxygen ion-conductive solid electrolytes such as ZrO₂. First and secondlayers from the bottom are designated as first and second substratelayers 16 a, 16 b respectively. Third and fifth layers from the bottomare designated as first and second spacer layers 16 c, 16 erespectively. Fourth and sixth layers from the bottom are designated asfirst and second solid electrolyte layers 16 d, 16 f respectively.

A space (reference gas-introducing space) 18, into which a reference gassuch as atmospheric air to be used as a reference for measuring oxidesis introduced, is formed between the second substrate layer 16 b and thefirst solid electrolyte layer 16 d, the space 18 being comparted by alower surface of the first solid electrolyte layer 16 d, an uppersurface of the second substrate layer 16 b, and side surfaces of thefirst spacer layer 16 c.

The second spacer layer 16 e is interposed between the first and secondsolid electrolyte layers 16 d, 16 f. First and second diffusionrate-determining sections 20, 22 are also interposed between the firstand second solid electrolyte layers 16 d, 16 f.

A first chamber 24 for adjusting the partial pressure of oxygen in ameasurement gas is formed and comparted by a lower surface of the secondsolid electrolyte layer 16 f, side surfaces of the first and seconddiffusion rate-determining sections 20, 22, and an upper surface of thefirst solid electrolyte layer 16 d. A second chamber 26 for finelyadjusting the partial pressure of oxygen in the measurement gas andmeasuring oxides such as nitrogen oxides (NOx) in the measurement gas isformed and comparted by a lower surface of the second solid electrolytelayer 16 f, a side surface of the second diffusion rate-determiningsection 22, a side surface of the second spacer layer 16 e, and an uppersurface of the first solid electrolyte layer 16 d.

The external space communicates with the first chamber 24 via the firstdiffusion-rate determining section 20, and the first chamber 24communicates with the second chamber 26 via the second diffusionrate-determining section 22.

The first and second diffusion-rate determining sections 20, 22 givepredetermined diffusion resistances to the measurement gas to beintroduced into the first and second chambers 24, 26 respectively. Eachof the first and second diffusion-rate determining sections 20, 22 canbe formed as a passage composed of, for example, a porous material (forexample, a porous member composed of ZrO₂), or a small hole having apredetermined cross-sectional area so that the measurement gas may beintroduced. Alternatively, each of the first and second diffusion-ratedetermining sections 20, 22 may be constructed by a gap layer or aporous layer produced by printing. In this embodiment, the comparativemagnitude does not matter between the respective diffusion resistancesof the first and second diffusion rate-determining sections 20, 22.However, it is preferable that the diffusion resistance of the seconddiffusion rate-determining section 22 is larger than that of the firstdiffusion rate-determining section 20.

The atmosphere in the first chamber 24 is introduced into the secondchamber 26 under the predetermined diffusion resistance via the seconddiffusion rate-determining section 22.

An inner pumping electrode 28 having a substantially rectangular planarconfiguration and composed of a porous cermet electrode is formed on anentire lower surface portion for forming the first chamber 24, of thelower surface of the second solid electrolyte layer 16 f. An outerpumping electrode 30 is formed on a portion corresponding to the innerpumping electrode 28, of the upper surface of the second solidelectrolyte layer 16 f. An electrochemical pumping cell, i.e., a mainpumping cell 32 is constructed by the inner pumping electrode 28, theouter pumping electrode 30, and the second solid electrolyte layer 16 finterposed between the both electrodes 28, 30.

A desired control voltage (pumping voltage) Vp0 is applied between theinner pumping electrode 28 and the outer pumping electrode 30 of themain pumping cell 32 by the aid of an external variable power source 34to allow a pumping current Ip0 to flow in a positive direction or in anegative direction between the outer pumping electrode 30 and the innerpumping electrode 28. Thus, the oxygen in the atmosphere in the firstchamber 24 can be pumped out to the external space, or the oxygen in theexternal space can be pumped into the first chamber 24.

A measuring electrode 36 having a substantially rectangular planarconfiguration and composed of a porous cermet electrode is formed in theclose vicinity of the second diffusion rate-determining section 22 on anupper surface portion for forming the first chamber 24, of the uppersurface of the first solid electrolyte layer 16 d. A reference electrode38 is formed on a lower surface portion exposed to the referencegas-introducing space 18, of the lower surface of the first solidelectrolyte layer 16 d. An electrochemical sensor cell, i.e., acontrolling oxygen partial pressure-detecting cell 40 is constructed bythe measuring electrode 36, the reference electrode 38, and the firstsolid electrolyte layer 16 d.

The controlling oxygen partial pressure-detecting cell 40 is operatedsuch that the partial pressure of oxygen in the atmosphere in the firstchamber 24 can be detected by measuring the electromotive force V1generated between the measuring electrode 36 and the reference electrode38 by using a voltmeter 42, on the basis of the difference in oxygenconcentration between the atmosphere in the first chamber 24 and thereference gas (atmospheric air) in the reference gas-introducing space18.

The detected value of the partial pressure of oxygen is used to controlthe pumping voltage Vp0 of the variable power source 34 by the aid of afeedback control system 44. Specifically, the pumping operation effectedby the main pumping cell 32 is controlled so that the partial pressureof oxygen in the atmosphere in the first chamber 24 has a predeterminedvalue which is sufficiently low to make it possible to perform thecontrol of the partial pressure of oxygen in the second chamber 26 inthe next step. Each of the inner pumping electrode 28, the outer pumpingelectrode 30, and the measuring electrode 36 is composed of an inertmaterial having a low catalytic activity on NOx, for example, NOcontained in the measurement gas introduced into the first chamber 24.

Specifically, each of the inner pumping electrode 28 and the outerpumping electrode 30 may be composed of a porous cermet electrode. Inthis embodiment, each of the electrodes is composed of a metal such asPt and a ceramic such as ZrO₂. Especially, it is necessary to use amaterial which has a weak reducing ability or no reducing ability withrespect to the NO component in the measurement gas, for the innerpumping electrode 28 and the measuring electrode 36 disposed in thefirst chamber 24 to make contact with the measurement gas. It ispreferable that each of the inner pumping electrode 28 and the measuringelectrode 36 is composed of, for example, a compound having theperovskite structure such as La₃CuO₄, a cermet comprising a ceramic anda metal such as Au having a low catalytic activity, or a cermetcomprising a ceramic, a metal of the Pt group, and a metal such as Auhaving a low catalytic activity. When an alloy composed of Au and ametal of the Pt group is used as an electrode material, it is preferableto add Au in an amount of 0.03 to 35% by volume of the entire metalcomponent.

On the other hand, an auxiliary pumping electrode 50 having asubstantially rectangular planar configuration and composed of a porouscermet electrode is formed on an entire lower surface portion forforming the second chamber 26, of the lower surface of the second solidelectrolyte layer 16 f. An auxiliary electrochemical pumping cell, i.e.,an auxiliary pumping cell 52 is constructed by the auxiliary pumpingelectrode 50, the reference electrode 38, the second solid electrolytelayer 16 f, the second spacer layer 16 e, and the first solidelectrolyte layer 16 d.

A desired constant voltage Vp2 is applied between the auxiliary pumpingelectrode 50 and the reference electrode 38 of the auxiliary pumpingcell 52 by the aid of an external power source 54. Thus, the oxygen inthe atmosphere in the second chamber 26 can be pumped out to thereference gas-introducing space 18. Accordingly, the partial pressure ofoxygen in the atmosphere in the second chamber 26 is controlled to havea low value of partial pressure of oxygen which does not substantiallyaffect the measurement for the amount of the objective component under acondition in which the measurement gas component (NOx) is notsubstantially reduced or decomposed. In this arrangement, the change inamount of oxygen introduced into the second chamber 26 is greatlyreduced as compared with the change in the measurement gas, owing to theoperation of the main pumping cell 32 for the first chamber 24.Accordingly, the partial pressure of oxygen in the second chamber 26 iscontrolled accurately and constantly.

The gas sensor 10A according to the first embodiment further comprises adetecting electrode 60 having a substantially rectangular planarconfiguration and composed of a porous cermet electrode. The detectingelectrode 60 is formed on an upper surface portion for forming thesecond chamber 26, separated from the second diffusion rate-determiningsection 22, of the upper surface of the first solid electrolyte layer 16d.

An electrochemical sensor cell, i.e., a measuring oxygen partialpressure-detecting cell 61 is constructed by the detecting electrode 60,the reference electrode 38, and the first solid electrolyte layer 16 d.An electrochemical pumping cell, i.e., a measuring pumping cell 62 isconstructed by the detecting electrode 60, the outer pumping electrode30, the first solid electrolyte layer 16 d interposed between the bothelectrodes 60, 30, the second spacer layers 16 e, and the second solidelectrolyte layer 16 f.

The detecting electrode 60 is composed of, for example, a porous cermetcomprising Rh as a metal capable of reducing NOx as the measurement gascomponent and zirconia as a ceramic. Accordingly, the detectingelectrode 60 functions as a NOx-reducing catalyst for reducing NOxexisting in the atmosphere in the second chamber 26.

In this embodiment, an electromotive force (electromotive force of theoxygen concentration cell) V2, which depends on the difference in oxygenconcentration between the atmosphere around the detecting electrode 60and the atmosphere around the reference electrode 38, is generatedbetween the detecting electrode 60 and the reference electrode 38.

Therefore, in the gas sensor 10A according to the first embodiment, theelectromotive force-measuring circuit 70 measures constantly theelectromotive force V2 generated between the detecting electrode 60 andthe reference electrode 38. The current supply circuit 14 controls thefrequency of the pulse-shaped current signal Sif corresponding to adifference between an the electromotive force V2 measured by theelectromotive force-measuring circuit 70 and the comparing voltage Vb.The pulse-shaped current signal Sif which is frequency-controlled on thebasis of the electromotive force V2, is allowed to flow from the outerpumping electrode 30 to the detecting electrode 60. The measuring system64 converts at least the frequency of the pulse-shaped current signalSif into a concentration of NOx.

Accordingly, the partial pressure of oxygen in the atmosphere around thedetecting electrode 60, in other words, the partial pressure of oxygendefined by the oxygen produced by the reduction or decomposition of themeasurement gas component (NOx) is detected as an electric signal bymeasuring the electromotive force V2 generated between the detectingelectrode 60 and the reference electrode 38 by using a measuring system64 by the aid of the current supply circuit 14.

A first external output terminal φo1 electrically connected to thedetecting electrode 60, a second external output terminal φo2electrically connected to the reference electrode 38 and a thirdexternal output terminal φo3 electrically connected to the outer pumpingelectrode 30 are led to the outside of the main sensor device 12respectively. As shown in FIG. 3, the first and second external outputterminals φo1, φo2 are connected to input terminals of the electromotiveforce-measuring circuit 70 respectively. The first and third externaloutput terminals φo1, φo3 are connected to first and second inputterminal φi1, φi2 of the current supply circuit 14 respectively. Thus,the main sensor device 12, the electromotive force-measuring circuit 70and the current supply circuit 14 are electrically connected to oneanother.

As shown in FIG. 2, the gas sensor 10A according to this embodimentfurther comprises a heater 66 for generating heat in accordance withelectric power supply from the outside. The heater 66 is embedded in aform of being vertically interposed between the first and secondsubstrate layers 16 a, 16 b. The heater 66 is provided in order toincrease the conductivity of oxygen ion. An insulative layer 68 composedof alumina or the like is formed to cover upper and lower surfaces ofthe heater 66 so that the heater 66 is electrically insulated from thefirst and second substrate layers 16 a, 16 b.

The heater 66 is arranged over the entire portion ranging from the firstchamber 24 to the second chamber 26. Accordingly, each of the firstchamber 24 and the second chamber 26 is heated to a predeterminedtemperature. Simultaneously, each of the main pumping cell 32, thecontrolling oxygen partial pressure-detecting cell 40, and the measuringpumping cell 62 is also heated to a predetermined temperature andmaintained at that temperature.

As shown in FIG. 3, the current supply circuit 14 of the gas sensor 10Aaccording to the first embodiment comprises a comparing circuit 74 fordetermining a difference between the electromotive force V2 measured bythe electromotive force-measuring circuit 70 and a comparing voltage Vb(for example, 450 mV) supplied from a comparing voltage-generatingcircuit 72 and amplifying the difference with a predetermined gain (forexample, 100 times) to make an output as a measured voltage Vc, arectangular wave-generating circuit 76 for outputting a rectangular wavesignal Sf having a frequency corresponding to the level of the measuredvoltage Vc supplied from the comparing circuit 74, and a driving circuit80 for performing ON-OFF control for a constant current i supplied froma constant current source 78, on the basis of the rectangular wavesignal Sf supplied from the rectangular wave-generating circuit 76.

An output terminal of the driving circuit 80 is electrically connectedto a negative terminal of the power supply 81. A positive terminal ofthe power supply 81 is electrically connected to the outer pumpingelectrode 30 via the second input terminal φi2 of the current supplycircuit 14 and the third external output terminal φo3 of the main sensordevice 12.

The rectangular wave-generating circuit 76 comprises an oscillating unit82 for generating a rectangular wave having a predetermined crest valueand having a predetermined pulse width, and a frequency-converting unit84 for controlling an oscillation frequency of the oscillating unit 82corresponding to the level of the measured voltage Vc supplied from thecomparing circuit 74. The rectangular wave signal Sf, which has thefrequency on the basis of the value of the electromotive force V2, isobtained from the rectangular wave-generating circuit 76.

In this embodiment, the pulse width of the rectangular wave is fixed tobe, for example, 10 μsec. The crest value has a level necessary toperform ON-OFF control for the constant current i by the aid of thedriving circuit 80. The circuit of the frequency-converting unit 84 isconstructed such that the lower the value of the electromotive force V2is, as with regard to the level of the comparing voltage Vb, the higherthe frequency is.

The constant current source 78 and the driving circuit 80 are connectedbetween the negative terminal of the power supply 81 and the first inputterminal φi1 connected to the detecting electrode 60. In the first solidelectrolyte layer 16 d, the second spacer layer 16 e and the secondsolid electrolyte layer 16 f, the constant current i is allowed to flowfrom the outer pumping electrode 30 to the detecting electrode 60 onlyduring a period of ON control effected by the driving circuit 80 (i.e.,during a period corresponding to the pulse width of the rectangular wavesignal Sf).

In other words, the ON-OFF control for the constant current i effectedby the driving circuit 80 provides a pulse-shaped current signal Sif inwhich the crest value is a predetermined value (for example, 500 μA)during the period corresponding to the pulse width of the rectangularwave signal Sf, and the crest value is, for example, 0 μA during theother periods (see a waveform “b” shown in FIG. 5).

Therefore, the constant current i flows from the reference electrode 38to the detecting electrode 60 during the period corresponding to thepulse width of the rectangular wave signal Sf outputted from therectangular wave-generating circuit 76. The oxygen, which is in anamount corresponding to a quantity of electricity represented by thecrest value (for example, 500 μA) of the constant current i×the pulsewidth of the rectangular wave signal Sf, is pumped from the secondchamber 26 to the external space.

The pumping operation causes a change in partial pressure of oxygen inthe second chamber 26. The change is measured as the electromotive forceV2 between the detecting electrode 60 and the reference electrode 38 bythe aid of the electromotive force-measuring circuit 70. The rectangularwave signal Sf, which has a frequency corresponding to the electromotiveforce V2, is supplied to the driving circuit 80. Thus, the constantcurrent i flows from the outer pumping electrode 30 to the detectingelectrode 60 during the period corresponding to the pulse width of therectangular wave signal Sf.

A characteristic shown in FIG. 4 represents a relationship between theNO concentration and the frequency of the rectangular wave signal Sfconcerning the gas sensor 10A according to the first embodiment.According to the characteristic, it is understood that the frequency islinearly increased in response to the NO concentration, making itpossible to measure the NO concentration.

On the other hand, as shown in FIG. 3, for example, two types ofcircuits are conceived for the measuring system 64. The first measuringsystem 64 a comprises a resistor R1 for extracting, as a voltage signalVi, the constant current i subjected to the ON-OFF control effected bythe driving circuit 80, a frequency-detecting circuit 90 for detectingthe frequency of the voltage signal Vi extracted by the aid of theresistor R1, and an output circuit 92 for converting the frequency valuedetected by the frequency-detecting circuit 90 on the basis of, forexample, the characteristic shown in FIG. 4 into the NO concentration sothat the concentration value is displayed, for example, by digitalexpression. The second measuring system 64 b comprises an output circuit94 for converting the measured voltage Vc supplied from the comparingcircuit 74 into the NO concentration so that the concentration value isdisplayed, for example, by digital expression.

The second measuring system 64 b is available because of the followingreason. That is, the frequency may be measured by detecting the timingof the flow of the constant current i to measure the frequency thereof,as performed in the first measuring system 64 a. However, the voltagewhich enters the frequency-converting unit of the rectangularwave-generating circuit 76, i.e., the measured voltage Vc based on thedifference between the comparing voltage Vb supplied from the comparingvoltage-generating circuit 72 and the electromotive force V2 between thedetecting electrode 60 and the reference electrode 38 directlyrepresents the frequency to be used for the control. The detection ofthe measured voltage Vc is equivalent to the measurement of thefrequency of the pulse-shaped current signal Sif. Especially, in thesecond measuring system 64 b, it is unnecessary to provide any circuitwhich is exclusively used to measure the frequency of the pulse-shapedcurrent signal Sif, making it possible to effectively simplify thecircuit arrangement.

The gas sensor 10A according to the first embodiment is basicallyconstructed as described above. Next, its operation and effect will beexplained.

At first, the electromotive force V2 between the reference electrode 38and the detecting electrode 60 of the gas sensor 10A is measured by theelectromotive force-measuring circuit 70. The electromotive force V2 iscompared with the comparing voltage Vb in the comparing circuit 74. Thecomparing circuit 74 determines the difference between the electromotiveforce V2 and the comparing voltage Vb. The difference is amplified withthe predetermined gain to be outputted as the measured voltage Vc.

The measured voltage Vc is introduced into the frequency-converting unit84 for adjusting the frequency of the rectangular wave signal Sfoutputted from the rectangular wave-generating circuit 76. Thefrequency-converting unit 84 controls the oscillation frequency of theoscillating unit 82 on the basis of the measured voltage Vc.Accordingly, the rectangular wave signal Sf is obtained, which has thefrequency based on the value of the electromotive force V2.

The rectangular wave signal Sf, which is outputted from the rectangularwave-generating circuit 76, is introduced into the driving circuit 80.The driving circuit 80 performs the ON-OFF control for the constantcurrent i supplied from the constant current source 78, on the basis ofthe rectangular wave signal Sf. The process is performed such that theconstant current i is allowed to flow during only the periodcorresponding to the pulse width of the rectangular wave signal Sf, andthe flow of the constant current i is stopped during the other periods.Accordingly, the pulse-shaped signal Sif flows from the referenceelectrode 38 to the detecting electrode 60.

In the case of the first measuring system 64 a, the frequency of thevoltage signal Vi detected by the resistor R1 is detected by thefrequency-detecting circuit 90. The frequency value detected by thefrequency-detecting circuit 90 is converted into the NOx concentrationby the output circuit 92, and it is displayed, for example, by digitalexpression. In the case of the second measuring system 64 b, themeasured voltage Vc supplied from the comparing circuit 74 is convertedinto the NOx concentration by the output circuit 94, and it isdisplayed, for example, by digital expression.

As described above, in the gas sensor 10A according to the firstembodiment, the pulse-shaped current signal Sif, which isfrequency-controlled on the basis of the electromotive force V2generated between the detecting electrode 60 and the reference electrode38, is allowed to flow from the outer pumping electrode 30 to thedetecting electrode 60. Therefore, the following effect can be obtained.

In the case of the conventional measuring method, for example, aconcentration of 1000 ppm can be merely detected with a low currentvalue in which the pumping current of the gas sensor is 5 μA. The systemtends to be affected by the external electric noise because the currentis small. However, in the case of the gas sensor 10A according to thefirst embodiment, the measuring system 64 (for example, the firstmeasuring system 64 a) is used to measure the frequency of thepulse-shaped current signal Sif having the crest value of 500 μA.Therefore, for example, when the critical value is set to be 250 μA tomeasure the frequency of the current signal Sif, it is possible toaccurately measure the NOx concentration, for example, even if the noisecomponent exists in an amount corresponding to 100 μA.

Next, a specified example of the gas sensor 10A according to the firstembodiment described above will be explained while making comparisonwith a case in which a direct current is allowed to flow from thereference electrode 38 to the detecting electrode 60.

At first, when the direct current is allowed to flow, for example, thedirect current is 5 μA for a concentration of NO of 1000 ppm as shown ina waveform “a” in FIG. 5.

Assuming that the period of time is 1 sec, the quantity of electricityis 5 μA·sec=5μ coulombs when the direct current flows. On the otherhand, in the gas sensor 10A according to the first embodiment, thepulse-shaped current signal Sif (see a waveform “b”), which has aquantity of electricity equivalent to the quantity of electricity (5μcoulombs), is allowed to flow from the outer pumping electrode 30 to thedetecting electrode 60. Simultaneously, for example, thefrequency-detecting circuit 90 is used to count the number of pulses ofthe voltage signal Vi per unit time. In other words, the frequency ofthe pulse of the current signal Sif (exactly, the rectangular wavesignal Sf) is controlled so as to provide the same value as the directcurrent value of the direct current (5 μA)×unit time (1 sec)

In the case of the specified example described above, the quantity ofelectricity possessed by one pulse is 10 μsec×500 μA=5000×10⁻⁶μcoulombs=5×10⁻³μ coulombs which is 1/1000 of that used for the directcurrent. Therefore, when 1000 individuals of pulses are allowed to flowfor 1 sec, i.e., when the pulse-shaped current signal Sif having afrequency of 1 kHz is allowed to flow, then it is possible to performthe aimed pumping operation (the pumping operation for making thepartial pressure of oxygen in the second chamber 26 to be the partialpressure of oxygen corresponding to the comparing voltage Vb).Simultaneously, it is possible to measure the NO concentration highlyaccurately without being affected by the electric noise.

Generally, some main sensor devices 12 have large limiting currents, andother main sensor devices 12 have small limiting currents, because of,for example, dispersion in production. When the same NOx concentrationis measured, the main sensor device 12 having a large limiting currentprovides the current signal Sif having a high frequency as compared withthe main sensor device 12 having a small limiting current. There is apossibility of occurrence of any measurement error.

In order to solve the foregoing problem, the conventional method hasrelied on, for example, the shunt resistor system or the voltage dividerresistor system. However, any of them has such an inconvenience that itis necessary to increase the number of lead wires, which isdisadvantageous in view of the cost.

A modified embodiment (10Aa) of the gas sensor 10A according to thefirst embodiment described below provides a gas sensor which makes itpossible to solve the problem as described above. The gas sensor 10Aawill be explained with reference to FIG. 6. Components or partscorresponding to those shown in FIG. 3 are designated by the samereference numerals.

As shown in FIG. 6, the gas sensor 10Aa according to the modifiedembodiment is constructed in approximately the same manner as in the gassensor 10A according to the first embodiment described above (see FIG.3). However, the former is different from the latter in that anadjusting resistor Rc is connected in series to the supply line for thecurrent signal Sif from the current supply circuit 14. In theillustrated embodiment, the adjusting resistor Rc is connected in seriesbetween the detecting electrode 60 and the first external outputterminal φo1 of the main sensor device 12.

It is now assumed a case in which the electric potential of the negativeterminal of the power supply 81 is set to be, for example, −5 V, thevoltage between the reference electrode 38 and the detecting electrode60 is, for example, 5 V, the value of the adjusting resistor Rc is setto be, for example, 10 kΩ, and the alternating current impedance betweenthe detecting electrode 60 and the outer pumping electrode 30 of themain sensor device 12 is set to be about 300 Ω.

The direct current impedance between the detecting electrode 60 and theouter pumping electrode 30 is about 2 kΩ. However, in the case of thealternating current having a high frequency, for example, a frequency ofnot less than 10 kHz, the impedance is about ⅕ to 1/10 thereof. Theimpedance is also sufficiently small for the rectangular wave signalcontaining a lot of high frequency components, as compared with thosefor the direct current. That is, the impedance has a value in such adegree that it can be sufficiently neglected with respect to theadjusting resistor Rc.

Accordingly, the current signal Sif, which flows from the outer pumpingelectrode 30 to the detecting electrode 60 in accordance with thedriving operation of the current supply circuit 14, is a current signalSif with pulses having a crest value of 500 μA. The crest value isdetermined by the size of the adjusting resistor Rc.

Therefore, the following operation is available for a gas sensor inwhich the main sensor device 12 has a large limiting current, forexample, for a gas sensor in which, for example, a direct current of 7μA is allowed to flow, for example, for a NOx concentration of 1000 ppm,when the measurement is performed by supplying the direct current. Thatis, when the resistance value of the adjusting resistor Rc is lowered,and the value of the constant current i flowing between the outerpumping electrode 30 and the detecting electrode 60 is set to be, forexample, 700 μA, then such a sensor behaves equivalently to a gas sensorin which a current of 5 μA is allowed to flow for the NOx concentrationof 1000 ppm.

As described above, in the gas sensor 10Aa according to the modifiedembodiment, the relationship between the NOx concentration and the pulsefrequency can be consequently maintained to be constant only by changingthe value of the adjusting resistor Rc, irrelevant to the dispersion(for example, any dispersion in sensitivity) among individual mainsensor devices 12. Thus, it is unnecessary to adopt the conventionalshunt resistor system and the conventional voltage divider resistorsystem.

In other words, the gas sensor 10Aa according to the modified embodimentmakes it possible to compensate the dispersion (dispersion in crestvalue or output) among individual sensors without increasing the numberof lead wires and terminals, which is advantageous in view of theproduction cost.

Next, a gas sensor 10B according to a second embodiment will beexplained with reference to FIG. 7. Components or parts corresponding tothose shown in FIG. 3 are designated by the same reference numerals.

As shown in FIG. 7, the gas sensor 10B according to the secondembodiment is constructed in approximately the same manner as in the gassensor 10A according to the first embodiment described above (see FIG.3). However, the former is different from the latter in that therectangular wave-generating circuit 76 of the current supply circuit 14comprises an oscillating unit 100 for generating a rectangular wavehaving a predetermined crest value and having a predetermined pulsewidth, and a duty ratio-converting unit 102 for controlling the dutyratio (the ratio of ON/OFF time) of the pulse signal outputted from theoscillating unit 100, depending on the level of the measured voltage Vcsupplied from the comparing circuit 74. A rectangular wave signal Sd,which has the duty ratio based on the value of the electromotive forceV2, is obtained from the rectangular wave-generating circuit 76.

The frequency of the pulse signal outputted from the oscillating unit100 is fixed to be, for example, 100 Hz. The crest value has a levelnecessary to perform ON-OFF control for the constant current i by theaid of the driving circuit 80 disposed at the downstream stage. Thecircuit of the duty ratio-converting unit 102 is constructed such thatthe lower the value of the electromotive force V2 is, as with regard tothe level of the comparing voltage Vb, the higher the duty ratio is (thelonger the period of the pulse width Pw is (see FIG. 9)).

In other words, the ON-OFF control for the constant current i effectedby the driving circuit 80 provides a pulse-shaped current signal Sid inwhich the crest value is a predetermined value (for example, 100 μA)during the period corresponding to the pulse width Pw of the rectangularwave signal Sd, and the crest value is, for example, 0 μA during theother periods (see a waveform “d” shown in FIG. 9). Since the frequencyof the pulse signal outputted from the oscillating unit 100 is 100 Hz,the frequency of the current signal Sid is fixed to the same frequencyof 100 Hz.

A characteristic shown in FIG. 8 represents a relationship between theNO concentration and the duty ratio of the current signal Sid concerningthe gas sensor 10B according to the second embodiment. According to thecharacteristic, it is understood that the duty ratio (ON time) islinearly increased in response to the NO concentration, making itpossible to measure the NO concentration.

As for the measuring system 64, for example, two types of circuits areconceived in the second embodiment as well. As shown in FIG. 7, thefirst measuring system 64 a comprises a duty ratio-detecting circuit 104for detecting the duty ratio (for example, the pulse width Pw) of thevoltage signal Vi extracted by the aid of the resistor R1, and an outputcircuit 106 for converting the duty ratio detected by the dutyratio-detecting circuit 104 on the basis of, for example, thecharacteristic shown in FIG. 8 into the NO concentration so that theconcentration value is displayed, for example, by digital expression.

On the other hand, the second measuring system 64 b comprises an outputcircuit 94 for converting the measured voltage Vc supplied from thecomparing circuit 74 into the NO concentration so that the concentrationvalue is displayed, for example, by digital expression. The duty ratiomay be measured by detecting the voltage signal Vi to measure the dutyratio thereof, as performed in the first measuring system 64 a. However,the voltage which enters the duty ratio-converting unit 102 of therectangular wave-generating circuit 76, i.e., the measured voltage Vcbased on the difference between the comparing voltage Vb supplied fromthe comparing voltage-generating circuit 72 and the electromotive forceV2 between the detecting electrode 60 and the reference electrode 38directly represents the duty ratio. The detection of the measuredvoltage Vc is equivalent to the measurement of the duty ratio of thepulse-shaped current signal Sid. Therefore, in the second measuringsystem 64 b, it is unnecessary to provide any circuit which isexclusively used to measure the duty ratio of the voltage signal Vi,making it possible to effectively simplify the circuit arrangement.

The gas sensor 10B according to the second embodiment is basicallyconstructed as described above. Next, its operation and effect will beexplained.

At first, the electromotive force V2 between the reference electrode 38and the detecting electrode 60 of the gas sensor 10B is measured by theelectromotive force-measuring circuit 70. The electromotive force V2 iscompared with the comparing voltage Vb in the comparing circuit 74. Thecomparing circuit 74 determines the difference between the electromotiveforce V2 and the comparing voltage Vb. The difference is amplified withthe predetermined gain to be outputted as the measured voltage Vc.

The measured voltage Vc is introduced into the duty ratio-convertingunit 102 for adjusting the duty ratio of the rectangular wave signal Sdoutputted from the rectangular wave-generating circuit 76. The dutyratio-converting unit 102 controls the duty ratio (the pulse width Pw)of the pulse signal outputted from the oscillating unit 100, on thebasis of the measured voltage Vc. Accordingly, the rectangular wavesignal Sd is obtained, which has the duty ratio based on the value ofthe electromotive force V2.

The rectangular wave signal Sd, which is outputted from the rectangularwave-generating circuit 76, is introduced into the driving circuit 80.The driving circuit 80 performs the ON-OFF control for the constantcurrent i on the basis of the rectangular wave signal Sd. The process isperformed such that the constant current i is allowed to flow duringonly the period corresponding to the pulse width Pw of the rectangularwave signal Sd, and the flow of the constant current i is stopped duringthe other periods. Accordingly, the pulse-shaped signal Sid flows fromthe outer pumping electrode 30 to the detecting electrode 60.

In the case of the first measuring system 64 a, the duty ratio of thevoltage signal Vi detected by the resistor R1 is detected by the dutyratio-detecting circuit 104. The duty ratio detected by the dutyratio-detecting circuit 104 is converted into the NOx concentration bythe output circuit 106, and it is displayed, for example, by digitalexpression. In the case of the second measuring system 64 b, themeasured voltage Vc supplied from the comparing circuit 74 is convertedinto the NOx concentration by the output circuit 94, and it isdisplayed, for example, by digital expression.

As described above, in the gas sensor 10B according to the secondembodiment, the pulse-shaped current signal Sid, which is controlled forthe duty ratio on the basis of the electromotive force V2 generatedbetween the detecting electrode 60 and the reference electrode 38, isallowed to flow from the outer pumping electrode 30 to the detectingelectrode 60. Therefore, the following effect can be obtained.

In the case of the conventional measuring method, for example, aconcentration of 1000 ppm can be merely detected with a low currentvalue in which the pumping current of the gas sensor is 5 μA. The systemtends to be affected by the external electric noise because the currentis small. However, in the case of the gas sensor 10B according to thesecond embodiment, the measuring system 64 is used to measure the dutyratio of the pulse-shaped current signal Sid, i.e., the pulse width Pwwhich is the time, having the crest value of 100 μA at a frequency of,for example, 100 Hz. Therefore, the system is scarcely affected by thenoise, and it is possible to accurately measure the NOx concentration.

Next, a specified example of the gas sensor 10B according to the secondembodiment described above will be explained while making comparisonwith a case in which a direct current is allowed to flow from thereference electrode 38 to the detecting electrode 60.

At first, when the direct current is allowed to flow, for example, thedirect current is 5 μA for a concentration of NO of 1000 ppm as shown ina waveform “c” in FIG. 9.

Assuming that there is given one cycle=10 msec for the pulse signaloutputted from the oscillating unit 100, the quantity of electricity is5 μA·10 msec=50×10⁻³ μA·sec=50×10⁻³μ coulombs when the direct currentflows.

On the other hand, in the gas sensor 10B according to the secondembodiment, the pulse-shaped electric signal Sid, which has a quantityof electricity equivalent to the quantity of electricity (50×10⁻³μcoulombs), is allowed to flow from the reference electrode 38 to thedetecting electrode 60. Simultaneously, for example, the dutyratio-detecting circuit 104 is used to measure the pulse width Pw of thevoltage signal Vi. That is, the duty ratio of the pulse of the currentsignal Sid (exactly, the rectangular wave signal Sd), in other words,the pulse width Pw is controlled so as to provide the same value as thedirect current value of the direct current (5 μA)×unit time (10 msec).

In the case of the specified example described above, the pulse widthPw, which is equivalent to the quantity of electricity of 50×10⁻³μcoulombs is (50×10⁻³ μ·sec)/100 μA=0.5 msec. The duty ratio is 0.5msec/10 msec which is 1/20. Therefore, when the pulse signal having theduty ratio of 1/20, i.e., the pulse-shaped current signal Sid having thepulse width Pw of 0.5 msec is allowed to flow, then it is possible toperform the aimed pumping operation (the pumping operation for makingthe partial pressure of oxygen in the second chamber 26 to be thepartial pressure of oxygen corresponding to the comparing voltage Vb).Simultaneously, it is possible to measure the NO concentration highlyaccurately without being affected by the electric noise.

Next, a modified embodiment (10Ba) of the gas sensor 10B according tothe second embodiment will be explained with reference to FIG. 10.Components or parts corresponding to those shown in FIG. 7 aredesignated by the same reference numerals.

Some main sensor devices 12 have large limiting currents, and other mainsensor devices 12 have small limiting currents, because of, for example,dispersion in production. When the same NOx concentration is measured,the main sensor device 12 having a large limiting current provides ahigh duty ratio (a long pulse width Pw) as compared with the main sensordevice 12 having a small limiting current. There is a possibility ofoccurrence of any measurement error.

The gas sensor 10Ba according to the modified embodiment eliminates anymeasuring error which would be otherwise caused by the dispersion amongindividual main sensor devices 12. As shown in FIG. 10, the gas sensor10Ba according to the modified embodiment is constructed inapproximately the same manner as in the gas sensor 10B according to thesecond embodiment described above (see FIG. 7). However, the former isdifferent from the latter in that an adjusting resistor Rc is connectedin series to the supply line for the current signal Sid from the currentsupply circuit 14. In the illustrated embodiment, the adjusting resistorRc is connected between the detecting electrode 60 and the firstexternal output terminal φo1 of the main sensor device 12.

It is now assumed a case in which the electric potential of the negativeterminal of the power supply 81 is set to be, for example, −5 V, thevoltage between the reference electrode 38 and the detecting electrode60 is, for example, 5 V, the value of the adjusting resistor Rc is setto be, for example, 50 kΩ, and the alternating current impedance betweenthe detecting electrode 60 and the outer pumping electrode 30 of themain sensor device 12 is set to be about 300 Ω. The current, which flowsfrom the outer pumping electrode 30 to the detecting electrode 60 inaccordance with the driving operation of the current supply circuit 14,is a current signal Sid with pulses having a frequency of 100 Hz andhaving a crest value of 100 μA. The crest value is determined by thesize of the adjusting resistor Rc.

Therefore, the following operation is available for a gas sensor inwhich the main sensor device 12 has a large limiting current, forexample, for a gas sensor in which, for example, a direct current of 7μA is allowed to flow, for example, for a NOx concentration of 1000 ppm.That is, when the resistance value of the adjusting resistor Rc islowered, and the value of the constant current i flowing between theouter pumping electrode 30 and the detecting electrode 60 is set to be,for example, 140 μA, then such a sensor behaves equivalently to a gassensor in which a current of 5 μA is allowed to flow for the NOxconcentration of 1000 ppm.

As described above, in the gas sensor 10Ba according to the modifiedembodiment, the relationship between the NOx concentration and the pulsefrequency can be consequently maintained to be constant only by changingthe value of the adjusting resistor Rc, irrelevant to the dispersion(for example, any dispersion in sensitivity) among individual mainsensor devices 12. Thus, it is unnecessary to adopt the conventionalshunt resistor system and the conventional voltage divider resistorsystem.

In other words, the gas sensor 10Ba according to the modified embodimentmakes it possible to compensate the dispersion (dispersion in crestvalue or output) among individual sensors without increasing the numberof lead wires and terminals, which is advantageous in view of theproduction cost.

Next, a gas sensor 10C according to a third embodiment will be explainedwith reference to FIGS. 11 to 14. Components or parts corresponding tothose shown in FIG. 3 are designated by the same reference numerals.

As shown in FIG. 11, the gas sensor 10C according to the thirdembodiment is constructed in approximately the same manner as in the gassensor 10A according to the first embodiment described above (see FIG.3). However, the former is different from the latter in the arrangementof the current supply circuit 14 as follows.

That is, as shown in FIG. 11, the current supply circuit 14 comprises acomparing circuit 74 for determining a difference between theelectromotive force V2 measured by the electromotive force-measuringcircuit 70 and a comparing voltage Vb (for example, 450 mV) suppliedfrom a comparing voltage-generating circuit 72 and outputting it as avoltage signal Sa, an amplifying circuit 110 for amplifying the voltagesignal Sa supplied from the comparing circuit 74, for example, 500 timesto give a measured voltage signal Sv, a rectangular wave-generatingcircuit 112 for generating a pulse signal Sp having a frequency of 1 kHzand having a duty ratio of, for example, 1/1000 (ON period: 1 μsec, OFFperiod: 999 μsec), and a driving circuit 114 for performing ON-OFFcontrol for the measured voltage signal Sv supplied from the amplifyingcircuit 110, on the basis of the pulse signal Sp (rectangular wave)supplied from the rectangular wave-generating circuit 112.

An output line of the driving circuit 114 is electrically connected tothe second input terminal φi2 so that a current iv, which corresponds tothe voltage outputted from the driving circuit 114, is supplied to theouter pumping electrode 30. The current iv is detected as a voltage Viby the aid of a current-detecting resistor R2 (for example 10 kΩ)connected and inserted between the driving circuit 114 and the secondinput terminal φi2.

A pulse-shaped driving signal Svp (voltage signal), which has a constantfrequency and a constant duty ratio and which has a voltage level basedon the value of the electromotive force V2, is obtained from the drivingcircuit 114.

In other words, the ON-OFF control effected by the driving circuit 14for the output (the measured voltage signal Sv) from the amplifyingcircuit 110 allows the output from the driving circuit 114 to be thepulse-shaped driving signal Svp (the voltage signal) which has a crestvalue based on the electromotive force V2 during the periodcorresponding to the pulse width of the pulse signal Sp from therectangular wave-generating circuit 112 and which has a crest value of,for example 0 μA during the other periods (see a waveform “g” shown inFIG. 13). The current iv, which corresponds to the driving signal Svp,is supplied to the outer pumping electrode 30. Since the pulse signal Spoutputted from the rectangular wave-generating circuit 112 has thefrequency of 1 kHz, the frequency of the driving signal Svp is fixed tobe the same frequency of 1 kHz.

A characteristic shown in FIG. 12 represents a relationship between theNO concentration and the crest value (the voltage Vi obtained afterconversion of the current iv into the voltage) of the current ivconcerning the gas sensor according to the third embodiment (see a solidline A). According to the characteristic, it is understood that thecrest value (the voltage Vi) of the current iv is linearly increased inresponse to the NO concentration, making it possible to measure the NOconcentration.

As for the measuring system 64, for example, two types of circuits areconceived in the third embodiment as well. The first measuring system 64a comprises a voltage-detecting circuit 116 for detecting the voltage Vi(the voltage corresponding to the crest value of the current iv)extracted by the current-detecting resistor R2 to output its peak valueand its average value, and an output circuit 118 for converting theoutput (the peak value and the average value) from the voltage-detectingcircuit 116 on the basis of, for example, the characteristic shown inFIG. 12 into the NO concentration so that the concentration value isdisplayed, for example, by digital expression. The second measuringsystem 64 b comprises an output circuit 120 for converting the measuredvoltage signal Sv supplied from the amplifying circuit 110 into the NOconcentration so that the concentration value is displayed, for example,by digital expression.

The gas sensor 10C according to the third embodiment is basicallyconstructed as described above. Next, its operation and effect will beexplained.

At first, the electromotive force V2 between the reference electrode 38and the detecting electrode 60 of the gas sensor 10C is measured by theelectromotive force-measuring circuit 70. The electromotive force V2 iscompared with the comparing voltage Vb in the comparing circuit 74. Thecomparing circuit 74 outputs, as the voltage signal Sa, the differencebetween the electromotive force V2 and the comparing voltage Vb. Thevoltage signal Sa is amplified with the predetermined gain (for example,500 times) to give the measured voltage signal Sv by the aid of theamplifying circuit 110 disposed at the downstream stage.

The measured voltage signal Sv is introduced into the driving circuit114. The driving circuit 114 performs the ON-OFF control for theinputted measured voltage signal Sv on the basis of the pulse signal Spsupplied from the rectangular wave-generating circuit 112. Accordingly,the pulse-shaped driving signal Svp (the voltage signal) is obtained,which has the voltage level based on the value of the electromotiveforce V2. The current iv corresponding to the driving signal Svp issupplied to the outer pumping electrode 30. During this process, thecurrent iv, which is determined by the voltage of the driving signal Svpand the impedance between the outer pumping electrode 30 and thedetecting electrode 60, flows between the outer pumping electrode 30 andthe detecting electrode 60 during the period corresponding to the pulsewidth of the driving signal Svp. That is, the pulse-shaped current ivflows between the outer pumping electrode 30 and the detecting electrode60.

The voltage Vi (the crest value of the current iv), which is extractedby the current-detecting resistor R2, is detected by thevoltage-detecting circuit 116 of the first measuring system 64 a to makeoutput as the peak value or the average value thereof. The peak value orthe average value is converted into the NOx concentration by the outputcircuit 118 disposed at the downstream stage, and the concentration isdisplayed, for example, by digital expression. In the second measuringsystem 64 b, the measured voltage signal Sv from the amplifying circuit110 is converted into the NOx concentration by the output circuit 120,and the concentration is displayed, for example, by digital expression.

As described above, in the gas sensor 10C according to the thirdembodiment, the pulse-shaped current iv, the crest value of which iscontrolled on the basis of the electromotive force V2 generated betweenthe detecting electrode 60 and the reference electrode 38, is suppliedto the outer pumping electrode 30. Therefore, the following effect canbe obtained.

In the case of the conventional measuring method, for example, as shownby a broken line B in the characteristic curve shown in FIG. 12, thedetection can be performed by merely using the small change in which thevoltage (the pumping voltage) between the reference electrode 38 and thedetecting electrode 60 is 450 mV to 460 mV (the increment correspondingto the direct current impedance between the reference electrode 38 andthe detecting electrode 60, i.e., 10 kΩ×5 μA=10 mV) with respect to, forexample, the change in concentration of 0 to 1000 ppm. On the contrary,in the case of the gas sensor 10C according to the third embodiment, asshown by the solid line A in the characteristic curve shown in FIG. 12,the large change is obtained (the increment corresponding to thealternating current impedance between the outer pumping electrode 30 andthe detecting electrode 60, i.e., 300 Ω×5000 μA=1500 mV). Therefore, thesystem is scarcely affected by the noise, and it is possible toaccurately measure the NOx concentration.

Next, a specified example of the gas sensor 10C according to the thirdembodiment described above will be explained while making comparisonwith a case in which a direct current is supplied to the referenceelectrode 38. This description is illustrative of a case in whichcomparison is made by using the voltage Vi obtained after conversioninto the voltage for the current iv supplied to the reference electrode38.

At first, when the direct current is allowed to flow, a waveform “f” asshown in FIG. 13 is obtained. That is, the electromotive force of 450 mV+the amount of voltage drop based on the direct current impedancebetween the reference electrode 38 and the detecting electrode 60 andthe pumping current flowing between the reference electrode 38 and thedetecting electrode 60 of 2 kΩ×5 μA (1000 ppm)=450 mV+10 mV. Of thisvoltage, the voltage of 10 mV except for the amount of the electromotiveforce is a voltage which is substantially used for the oxygen pumping.

Assuming that there is given one cycle=1 msec for the pulse signal Spoutputted from the rectangular wave-generating circuit 112, the quantityof electricity is 5 μA×1 msec=5×10⁻³ μA·sec=5×10⁻³μ coulombs(corresponding to the amount of oxygen subjected to the pumping) whenthe direct current flows.

On the other hand, in the gas sensor 10C according to the thirdembodiment, the current iv, which corresponds to the pulse-shapeddriving signal Svp having a quantity of electricity equivalent to thequantity of electricity (5×10⁻³μ coulombs), is supplied to the outerpumping electrode 30. Simultaneously, for example, the voltage-detectingcircuit 116 is used to measure the crest value (the peak value or theaverage value) of the current iv after conversion into the voltage. Thatis, the crest value of the current iv is controlled so as to provide thesame value as the direct current value after the conversion into thevoltage for the direct current (10 mV)×unit time (1 msec).

In the case of the specified example described above, the quantity ofelectricity, which is equivalent of the quantity of electricity of5×10⁻³ μA·sec, is (V/R)×1 μsec. R represents the internal resistancebetween the outer pumping electrode 30 and the detecting electrode 60. Ris greatly different from the value of 2 kΩ obtained for the directcurrent, and it is 300 Ω as the alternating current impedance.Therefore, the quantity of electricity is (V/300)×1 μsec.

V is determined so that the quantity of electricity is equal to 5×10⁻³μA·sec. Accordingly, there is given V=5×10⁻³ μA·sec/(1/300)×1 μsec=1500mA·Ω=1500 mV.

Therefore, when the pulse-shaped driving signal (the voltage signal) Svphaving the frequency of 1 kHz, the pulse width of 1 μsec, and the crestvalue of 1500 mV is outputted from the driving circuit 114, then it ispossible to perform the aimed pumping operation (the pumping operationfor making the partial pressure of oxygen in the second chamber 26 to bethe partial pressure of oxygen corresponding to the comparing voltageVb). Simultaneously, it is possible to measure the NO concentrationhighly accurately without being affected by the electric noise.

Next, a modified embodiment (10Ca) of the gas sensor 10C according tothe third embodiment will be explained with reference to FIG. 14.Components or parts corresponding to those shown in FIG. 11 aredesignated by the same reference numerals.

As shown in FIG. 14, the gas sensor 10Ca according to the modifiedembodiment is constructed in approximately the same manner as in the gassensor 10C according to the third embodiment described above (see FIG.11). However, the former is different from the latter in that anadjusting resistor Rc is connected in series to the current supply linefrom the current supply circuit 14 to the outer pumping electrode 30. Inthe illustrated embodiment, the adjusting resistor Rc is connectedbetween the outer pumping electrode 30 and the second external outputterminal φo2 of the main sensor device 12.

According to the gas sensor 10Ca concerning the modified embodiment, thefollowing effects are obtained. That is, the relationship between theNOx concentration and the voltage of the driving signal Svp can bemaintained to be constant by adjusting the size of the adjustingresistor Rc in conformity with the dispersion among individualsconcerning the limiting current value of the main sensor device 12.Moreover, it is possible to further increase the change in voltage ofthe driving signal Svp with respect to the change in NOx concentration.

For example, it is assumed that the adjusting resistor Rc is 1 kΩ. Inthe case of the embodiment described above, the duty ratio of thedriving signal Svp is 1/1000. Therefore, the pumping current iv, whichflows between the outer pumping electrode 30 and the detecting electrode60, is obtained by multiplying 5 μA by 1000. That is, the pulse-shapedcurrent iv of 5 mA flows. The voltage drop of 5 V occurs in theadjusting resistor Rc. The voltage of the driving signal Svp isapproximately 5 V+1.5 V (the voltage drop caused by the internalresistance between the reference electrode 38 and the detectingelectrode 60)+450 mV (the electromotive force)=6.95 V.

The voltage is 450 mV when the NOx concentration is 0 ppm, while a largechange of 6.5 V can be used for the detection for the change of the NOxconcentration of 0 to 1000 ppm. The following assumption holds for thedispersion among individuals concerning the limiting current value ofthe main sensor device 12. That is, for example, it is assumed that themeasurement is performed by supplying a direct current to a main sensordevice 12 having a large limiting current. For example, there is giventhe adjusting resistor Rc=(6.5 V−300 Ω×7 mA)/7 mA=(4.4×1000/7)=629 Ω fora main sensor device 12 in which a direct current of 7 μA is allowed toflow for a NOx concentration of 1000 ppm. Thus, it is possible to allowthe voltage at 1000 ppm to be 6.5 V+450 mV=6.95 V.

As described above, the change in voltage Vi (i.e., the change in crestvalue of the current iv) with respect to the NOx concentration is thehigh voltage change which is 100 times that measured by using the directcurrent, by supplying, to the outer pumping electrode 30, the current ivcorresponding to the pulse-shaped driving signal Svp outputted from thedriving circuit 114. Thus, it is possible to obtain the effect that thesystem is scarcely affected by the electric noise. Additionally, it ispossible to effectively compensate the dispersion among individual mainsensor devices 12.

In the gas sensors 10A to 10C according to the first to thirdembodiments described above (including the respective modifiedembodiments), the pulse shape of the pulse signal outputted from therectangular wave-generating circuit 76, 112 is the rectangular wave.Besides, it is allowable to use any waveform including, for example,trapezoidal waves, triangular waves, and sinusoidal waves (half waves).

Preferably, the electromotive force-measuring circuit 70 is providedwith a smoothing circuit. In this embodiment, the smoothing circuitpreferably has a time constant τ which is not less than 10 times thepulse cycle of the pulse signal outputted from the rectangularwave-generating circuit 76, 112.

As for the lower limit value of the frequency of the pulse signaloutputted from the rectangular wave-generating circuit 76, 112, it ispreferable to provide a cycle of about 1/10 fold of the responseperformance required for the gas sensors 10A to 10C (including therespective modified embodiments). For example, when the requirement forthe response performance of the gas sensors 10A to 10C (including therespective modified embodiments) is 100 msec, it is preferable to use apulse signal having a cycle of 10 msec, i.e., not less than 100 Hz.Thus, it is possible to sufficiently smooth the electromotive force V2by using the smoothing circuit without deteriorating the responseperformance.

In the gas sensor 10A according to the first embodiment (including themodified embodiment), it is preferable that the pulse width of therectangular wave signal Sf outputted from the rectangularwave-generating circuit 76 is decreased as short as possible, because itis possible to set a high crest value for the constant current i.

Also in the gas sensor 10B according to the second embodiment (includingthe modified embodiment), it is preferable that the pulse width Pw ofthe rectangular wave signal Sd outputted from the rectangularwave-generating circuit 76 is decreased as short as possible, because itis possible to set a high crest value for the constant current i.

In the gas sensor 10C according to the third embodiment, when thedispersion among individual main sensor devices 12 is not correctedwithout providing the adjusting resistor Rc, it is preferable that thefrequency of the pulse signal Sp outputted from the rectangularwave-generating circuit 112 is low, because of the following reason.That is, the lower the frequency is, the higher the alternating currentimpedance is. Accordingly, the voltage change of the driving signal Svpis increased.

In the modified embodiment 10Ca of the gas sensor according to the thirdembodiment, the dispersion among individual main sensor devices 12 iscorrected by providing the adjusting resistor Rc. Therefore, it ispreferable that the frequency of the pulse signal Sp outputted from therectangular wave-generating circuit 112 is high, because of thefollowing reason. That is, the higher the frequency is, the lower thealternating current impedance is. Accordingly, the change in pumpingcurrent, which is determined by the value of the adjusting resistor Rc,is scarcely affected by the change in impedance of the measuring pumpingcell which depends on, for example, the change in temperature and thechange in durability.

The gas sensors 10A to 10C according to the first to third embodimentsdescribed above (including the respective modified embodiments) areillustrative of the case in which the NOx concentration in themeasurement gas is measured. However, the present invention is alsoapplicable, for example, to oxygen sensors, inflammable gas sensors, CO₂sensors, and H₂O sensors based on the use of the oxygen pump. Thepresent invention is also applicable to H₂ sensors and H₂O sensors basedon the use of the proton ion-conductive member, as well as tocontrolling pumps for controlling the concentration of such specifiedgases.

It is a matter of course that the gas sensor and the method forcontrolling the gas sensor according to the present invention are notlimited to the embodiments described above, which may be embodied inother various forms.

As explained above, according to the gas sensor and the method forcontrolling the gas sensor concerning the present invention, it ispossible to highly accurately measure a predetermined gas componentwhile scarcely being affected by the electric noise or the like.

Further, it is possible to compensate the dispersion among individualsensors without increasing the number of terminals, which isadvantageous in view of the production cost.

1. A gas sensor comprising: a main pumping means for pumping-processingoxygen contained in a measurement gas introduced from external space,comprising solid electrolyte contacting with said external space, and aninner pumping electrode and an outer pumping electrode formed on innerand outer surfaces of said solid electrolyte; and a measuring pumpingmeans for decomposing a predetermined gas component contained in saidmeasurement gas after being pumping-processed by said main pumping meansby the aid of a catalytic action and/or electrolysis, andpumping-processing oxygen produced by said decomposition via said outerpumping electrode of said main pumping means, wherein: a concentrationof oxygen is controlled and/or the predetermined gas component ismeasured by allowing a pulse-shaped current to flow through saidmeasuring pumping means; the gas sensor further comprising: aelectromotive force-measuring circuit for constantly measuring theelectromotive force corresponding to a difference between an amount ofoxygen produced by said decomposition of said predetermined gascomponent and an amount of oxygen contained in a reference gas; a crestvalue control means for controlling a crest value of said pulse-shapedcurrent corresponding to a difference between an the electromotive forcemeasured by said electromotive force-measuring circuit and a comparingvoltage; and a measuring circuit for at least converting the crest valueof the pulse-shaped current into a concentration of said predeterminedgas component.
 2. The gas sensor according to claim 1, wherein saidconcentration of oxygen is controlled and/or said predetermined gascomponent is measured by converting said pulse-shaped current having isconverted into a voltage to detect said crest value.
 3. The gas sensoraccording to claim 2, wherein a resistor is connected in series to asupply line of said pulse-shaped current to said measuring pumpingmeans.
 4. The gas sensor according to claim 3, wherein said resistor isselected or adjusted depending on performance of a sensor element. 5.The gas sensor according to claim 1, wherein said predetermined gascomponent is NOx.
 6. A method for controlling a gas sensor, the gassensor comprising: a main pumping means for pumping-processing oxygencontained in a measurement gas introduced from external space,comprising solid electrolyte contacting with said external space, and aninner pumping electrode and an outer pumping electrode formed on innerand outer surfaces of said solid electrolyte; and a measuring pumpingmeans for decomposing a predetermined gas component contained in saidmeasurement gas after being pumping-processed by said main pumping meansby the aid of a catalytic action and/or electrolysis, andpumping-processing oxygen produced by said decomposition via said outerpumping electrode of said main pumping means; wherein a concentration ofoxygen is controlled and/or the predetermined gas component is measuredby allowing a pulse-shaped current to flow through said measuringpumping means; wherein the method for controlling the gas sensorcomprises the steps of: measuring constantly the electromotive forcecorresponding to a difference between an amount of oxygen produced bysaid decomposition of said predetermined gas component and an amount ofoxygen contained in a reference gas; controlling a crest value of saidpulse-shaped current corresponding to a difference between an theelectromotive force measured by said electromotive force-measuringcircuit and a comparing voltage; and converting at least the crest valueof the pulse-shaped current into a concentration of said predeterminedgas component.
 7. The method for controlling said gas sensor accordingto claim 6, wherein said predetermined gas component is NOx.